This invention relates to a method for producing a self-alignment or self aligning type CCD image sensor. More particularly, to a method for producing a CCD image sensor wherein the effective area the light-receiving region is undiminished and incident light impinges only upon the light-receiving region, with the remaining regions of the sensor completely shielded from the incident light, thereby preventing smearing.
In general, a CCD image sensor is a dynamic element in which a signal charge moves along a predetermined route under the control of clock pulses, and the sensor generally comprises a light-receiving region and a transmission region for transferring a signal charge corresponding to incident light. The CCD image sensor has widely been used in memory, logic, signal processing or image processing devices.
One of the prior art CCD image sensors has a light-shielding layer of metal material such as aluminum for assuring that light can be incident only on the light-receiving region.
Such prior art CCD image sensors have been proposed by Mitsubishi Electronics Ind. Co., Ltd. in the Japanese TV Society brochure (Vol. 10, No. 4, Apr., 1990).
The process for producing the prior art CCD image sensor having a light-shielding layer of metal material is hereinafter described with reference to FIGS. 1(A) through (D).
As shown in FIG. 1(a), n-type ions are injected into a silicon substrate 1 at predetermined regions thereof at a predetermined distance to constitute a photo diode 2 for serving as a light-receiving region and a vertical charge coupled device (VCCD) for serving as a transmission region for transferring signal charges. The whole surface of the substrate 1 is covered with a gate oxide layer 4 and a gate polysilicon layer 5, the latter being over the former layer. Thereafter, the gate polysilicon layer 5 is removed from the portion over the photo diode 2 through conventional photo and etching processes.
Thereafter, as shown in FIG. 1(B), a Boron Phosphorous Silicate Glass (BPSG) film of a predetermined thickness for serving as an insulation film, is spread on the etched surface, and subsequently covered with a light-shielding layer 7 of aluminum of a predetermined thickness as shown in FIG. 1(C).
It is noted that a low temperature oxide (LTO) film can be used instead of the BPSG film.
The next step is to eliminate the light-shielding layer 7 only at the portion over the photo diode through conventional photo and etching processes for assuring that light is incident only upon the photo diode 2 (see FIG. 1(D)).
The reason why aluminum is typically used as the material for forming the light-shielding layer, is that aluminum is widely used in memory or bipolar elements and it is particularly suitable for use as a light-shielding layer because of its high reflectivity (approximately 100%).
However, the above-mentioned prior art suffers from the following disadvantages.
Firstly, elimination of the light-shielding layer 7 only from the region over the photo diode 2 results in the infiltration of light into the BCCD 3 through the remaining gate polysilicon layer 5, thus producing the appearance of smear (see FIG. 2).
Secondly, the stack structure of the gate polysilicon layer 5 and the light-shielding layer 7 makes it difficult to accomplish the photo and etching process for eliminating the light-shielding layer only at the portion over the photo diode 2, whereby the light-shielding layer 7 may extend into the region above the photo diode 2. Accordingly, the opening area of the photo diode 2 is decreased by 2Y, resulting in an undesirable decrease of the fill factor of the sensor.